msmtp: add extraAccounts option

It is appended to the end of msmtprc instead of top like extraConfig.

modules/programs/msmtp.nix

@@ -37,6 +37,8 @@ let
     ${cfg.extraConfig}
 
     ${concatStringsSep "\n\n" (map accountStr mailAccounts)}
+
+    ${cfg.extraAccounts}
   '';
 
 in {
@@ -53,6 +55,15 @@ in {
           See <link xlink:href="https://marlam.de/msmtp/msmtprc.txt"/> for examples.
         '';
       };
+
+      extraAccounts = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Extra configuration lines to add to the end of <filename>~/.msmtprc</filename>.
+          See <link xlink:href="https://marlam.de/msmtp/msmtprc.txt"/> for examples.
+        '';
+      };
     };
 
     accounts.email.accounts = mkOption {